Quench-hardening thermally hardenable steel



May 18, 1948. R. E. GRIFFITHS ET Al. 2,441,528

QUENCH-'HARDENING THERMALLYHARDENABLE STEEL v Filed Jan. 9, 1945 IW/f. Pfugvcf/ /90 E QUE/VCH.

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Patented May 18, 17948 UNITED isTATEs PATENT OFFICE QUENcn-HARDENING THERMALLY HARDENABLE STEEL Ronald E. Griffiths, Cleveland Heights, and

Gordon T. Spare, East Cleveland, Ohio, assignors to The American Steel and Wire Company of New Jersey, a corporation of New VJersey Application January 9, 1945, Serial No. 572,018

1 Claim. (Cl. 148-2155) This invention relates to the art of quenchhardening thermally hardenable steel.

Specifically, the invention is a method of hardening thermally hardenable steel by austenitizing the steel, quenching the austenitized steel in a quench at temperatures in the vicinity of the starting point of the martensite formation range, after its substantial temperature equalization in this quench and prior to its material austenite decomposition as quickly as possible removing the steel from this quench and quenching it in a quench at temperatures at about the lower limit of the martensite formationA range of the steel, and after its substantially complete production of martensite in this second-named quench as quickly as possible removing the steel therefrom, and immediately thereafter tempering it to the desired physical properties by reheating it.

It can be seen from the above that the invention resides in the lcharacterizing step, in the art of quench-hardening thermally hardenable steel, of interrupting the quench of the austenitized steel at about the temperature where martensite begins to form for a time effecting substantial temperature equalization throughout the steel Without material austenite decomposition, yfollowed by continuation of the quenching to develop martensite by the practice of quenching just through the martensite formation range and then immediately tempering.

In the first or interrupting quench the temperature of the steel becomes equalized at just about the point Where martensite begins to form, purely thermal stresses being largely eliminated and this, in conjunction with a reduction in the temperature differential between the steel and the subsequent or martensitizing quench, slowing the rate the austensite decomposes to martensite during this subsequent quench. During the martensitizing quench the formation of martensite proceeds smoothly with little chance for the forma-tion of microcracks or heavy stresses. Immediate tempering of the resulting martensitized steel provides it with ductility and toughness superior to steel quenched and tempered by the prior art methods and there will be less distortion. Y

The present invention is particularly applicable to the Vcontinuous production of tempered steel Wire, and the accompanying drawing graphically illustrates a comparison of wire tempered by the method of the present invention, by an example of the prior art method of continuously quenching just through the range of martensite formation and then immediately tempering, and by quenching through and below the range of martensite formation followed by immediate tempering. The drawing is suitably legended and needs little explanation beyond the following:

In the case of all curves steel wire containing .91% carbon, .42% manganese and otherwise of a plain carbon composition, of .080 inch diameter, was austenitized by being continuously passed for 10 seconds through lead maintained at 1650 F., the wire being then immediately continuously passed for seconds through oil maintained at F. in the case of the bottom curve for 60 seconds through oil maintained at 190 F. in the case of the middle curve and, in the case of the top curve which is exemplary of the present invention, the austenitized wire was immediately passed for 45 seconds through salt maintained at 400 F. and then immediately passed for 60 seconds through oil maintained at 190 F. It is to be noted that the middle and top curves differ only in that the top curve involves the interruption in the quenching that characterizes the present invention. In all cases the steps were sequential and as closely following as is possible with continuous steel wire tempering equipment.

In all cases the plotted points represent the averages of tests on three samples tempered to provide the tensile strengths indicated, the tempering having followed immediately after the quenching and having been conducted under comparable conditions in the case of all the curves.

It is to be noted that in the case of the bottom curve, representing the ordinary steel Wire tempering practice, the points are quite widely scattered and the curve can be drawn only to represent an indication of the trend. For the various tensile strengths, ductilities, as represented by the percent reduction of area, are low. In the case of the middle curve representing the better practice of quenching just through the range of martensite formation and then immediately tempering, the results are much more consistent and the ductilities are much better for the variousV tensile strengths. But it is clear from the top curve that in the case of the interrupted quench featuring the present invention the ductilities for the various tensile strengths are critically greater in the case of this invention.

Determination of the martensite point may be carried out by the technique described by A. B. Greninger and A. R. Troiano, Kinetics of the austenite to martensite transformation in steel, Transactions of the American Society for Metals (1940) 28, 537. The same technique will suffice to determine the ending of the range of rapid martenite formation.

We claim:

A method of hardening thermally hardenable steel by austenitizing the steel, quenching the `aust-enitized steel in a quench at temperatures in the vicinity of the martensite poi-nt oi thel steel, after its substantial temperature equalization. and' prior to its material austenite Vdecomposition immediately removing the steel from said quench and quenching it in a liquid quench the temperature of which is maintained at the lower limit of the range of martensite formation of the steel, and after its substantially complete production of martensite immediately removing the steel from the second-named quench, and then tempering it to the desired physical properties.

RONALD E. GRIFFITHS. GORDON T. SPARE.

REFERENCES CITED Jan. 28, 1943. 13D. 50-52, and Feb. 4, 1943, pp. 4548. 

